I Want an Office

When we moved in to this house in 2005, we had an office. It was in the guest room which is now MJ and RJ's bedroom. When MJ was about 1 1/2, I decided to have RJ and MJ share the larger room and started working from home. RJ's room became the guest room and MJ's nursery became my office. Fast forward about 6 months and I am pregnant with Molly. During her pregnancy it became clear that we would be using the guest room (RJ's old room) for her nursery and then having to convert the office (MJ's old nursery) in to the guest room. Whew, did you follow that? So, for 3 years, I have been missing an office. I know there has to be a way to make a 10X10 room a usable space both for my office as well as for overnight guests. I have contemplated a (nicer) futon but have those completely gone out of style? I also considered converting our guest room closet in to an office via Pinterest but I don't know how feasible that would be as well. I welcome any tips or suggestions you can give!